<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>内边框</title>
    <style>
        .con{
            width: 1000px;
            height: 600px;
            border: 3px solid red;
            padding: 20px;
        }
        .box{
            width: 200px;
            height: 100px;
            border: 3px solid blue;
            margin: 20px;
        }
    </style>
</head>
<body>
    <div class="con">
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
</body>
</html>